The American Civil Liberties Union is suing Cranston on its decision to count state prison inmates on its voter rolls. The suit, filed Wednesday, said that a quarter of the 13,000 people counted as residents in Cranston’s Sixth Ward are inmates who cannot vote there. When the 3,400 inmates at the Adult Correctional Institutions are subtracted, the Sixth Ward has fewer residents than other city wards, yet it has the same level of representation on the City Council and School Committee. The ACLU suit alleges that the city is violating constitutional requirements that citizens have equal representation.